In certain types of integrated circuit fabrication circuit patterns on a mask are transferred to a semiconductor wafer by using a source of light or other form of radiation to project the circuit patterns onto the wafer which is coated with a light responsive photoresist material. The light is transmitted through an appropriate optical arrangement which among other things serves the function of focusing the light onto the wafer so that the circuit pattern lines are well defined as projected onto the wafer. In order to provide a high-resolution image on the wafer it is important that the wafer be accurately positioned within the focal plane of the projection optical system. Thus, a problem associated with photolithographic projection systems wherein an exposed wafer is constantly being replaced by an unexposed wafer is the accurate positioning of the wafer to be exposed in the focal plane of the optical projection system. This problem exists even in those systems where the wafer mount, projection optics and light source remain fixed because wafers are not flat or of uniform thickness and even the slightest deviation from the focal plane gives undesirable results.
Another problem associated with photolithographic projection systems is caused by the structural changes in the silicon wafers caused by the diffusion, oxidation and epitaxial processes used to create the circuit elements. Thus, a typical silicon wafer has an imperfectly flat surface which may contain a number of undulations. Such deviations from a perfectly flat plane also lessen the resolution of circuit patterns projected onto such a surface.
The present invention relates to a system which automatically positions the wafer in the focal plane of the projection system and automatically flattens the surface of the wafer.